Remove builder API, clean up, make py3 only

[lorencarvalho]

In this PR I've removed the builder API, did some housekeeping of tox/flake/mypy/etc tasks and made the library Python 3 exclusive

Resolving the merge conflicts shouldn't be too bad, I'm happy to do that.

LMK what you think @epeay

[lorencarvalho]

note to self: update the readme & docs

[lorencarvalho]

accidentally closed :x

[catern]

Just a drive-by comment since I saw this PR, but why would you want to remove the builder API? That seems like the main value proposition of using this library instead of https://github.com/peakwinter/python-nginx. Is there something wrong with the builder API?

[lorencarvalho]

@catern

Hi! No worries re: driveby comments, always happy to oblige. We're not currently using the builder API internally (at LinkedIn), so this change is part of an effort to port the changes from our internal fork to OSS. I'd like to get to a point where we aren't maintaining the internal fork at all and we can just use this version.

I was not aware of python-nginx, but that project looks really nice. I especially like that it can parse configs as well as represent them as code. That's actually something I wanted to do with this library as well. I will probably do some experimenting with it and see if it works for our use cases in which case this library could simply be deprecated in favor of that one, though I've heard that the viral nature of GPLv3 could complicate things, though I admit I'm not actually well versed in licensing laws (I don't mean to spread FUD).